# Lecture 7: Peritoneum and Peritoneal Cavity ## Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the structure and function of the peritoneum 2. Differentiate between intraperitoneal and retroperitoneal organs 3. Identify the major peritoneal folds (mesenteries, omenta, ligaments) 4. Describe the divisions of the peritoneal cavity and their clinical significance 5. Explain the peritoneal recesses and their importance in fluid accumulation 6. Describe the blood supply, lymphatic drainage, and innervation of the peritoneum --- ## Overview of the Peritoneum The peritoneum is the largest serous membrane in the body, with a surface area of approximately 1.8 square meters, comparable to the skin. This extensive membrane lines the abdominal cavity and covers the abdominal viscera, secreting serous fluid that lubricates the surfaces and allows organs to glide smoothly against each other during peristalsis and respiration. Structurally, the peritoneum consists of a single layer of simple squamous mesothelium overlying a thin layer of connective tissue. The mesothelial cells produce the serous fluid that keeps the peritoneal surfaces moist and friction-free. The peritoneum exists as two continuous layers. The parietal peritoneum lines the abdominal and pelvic walls, attaching firmly to the body wall structures. It receives somatic innervation, which means pain arising from this layer is sharp and well-localized to the area of irritation. The blood supply to the parietal peritoneum comes from the same vessels that supply the body wall. The visceral peritoneum covers the abdominal organs, investing them to varying degrees. This layer receives autonomic innervation, so pain originating from visceral peritoneum is poorly localized and often perceived in the midline. Blood supply to the visceral peritoneum derives from the visceral vessels supplying the organs it covers. The peritoneal cavity is the potential space between the parietal and visceral layers. It normally contains only a thin film of serous fluid, approximately 50-100 milliliters. In males, the peritoneal cavity is completely closed. In females, however, it communicates with the exterior through the uterine tubes, uterine cavity, and vagina, providing a potential route for infection to enter the peritoneal space. Panel A: Parietal peritoneum lining the anterior and posterior abdominal walls, receiving somatic innervation for well-localized pain sensation. Panel B: Visceral peritoneum covering the abdominal organs including liver, stomach, and intestines, receiving autonomic innervation. Panel C: The peritoneal cavity as a potential space between the layers containing lubricating serous fluid, with greater and lesser sacs labeled. Panel D: Magnified inset showing the mesothelial cell layer with underlying connective tissue that produces serous fluid, with 5 cm scale bar. --- ## Organ Relationships to Peritoneum Abdominal organs are classified according to their relationship with the peritoneum, which determines their mobility and surgical accessibility. Intraperitoneal organs are completely or nearly completely covered by visceral peritoneum. They are suspended from the body wall by mesenteries and have considerable mobility within the abdominal cavity. The intraperitoneal organs include the stomach, spleen, liver (mostly covered), gallbladder, jejunum and ileum of the small intestine, cecum and appendix, transverse colon, sigmoid colon, superior portion of the rectum, and in females, the ovaries and uterine tubes. Despite the terminology, these organs are not actually inside the peritoneal cavity but are invaginated into it, covered by visceral peritoneum. Retroperitoneal organs lie behind the peritoneum, with only their anterior surface covered by parietal peritoneum. These organs are fixed to the posterior abdominal wall and are less mobile. Retroperitoneal organs are subdivided into two categories. Primary retroperitoneal organs never had a mesentery during development and have always been in this position. These include the kidneys, ureters, suprarenal glands, abdominal aorta, inferior vena cava, and the middle and lower portions of the rectum. Secondary retroperitoneal organs originally had a mesentery during fetal development but subsequently fused with the posterior abdominal wall. A helpful mnemonic is "SAD PUCKER": the Second part of the duodenum, Ascending colon, Descending colon, Pancreas (head and body), the rectum (upper part), and the sometimes-retroperitoneal cecum. Some organs in the pelvis lie below the peritoneum in a subperitoneal or infraperitoneal position. These include the bladder, the lower rectum, and parts of the uterus. Panel A: Intraperitoneal organs including stomach, liver, small intestine, and transverse colon suspended by mesenteries and completely covered by visceral peritoneum. Panel B: Primary retroperitoneal organs (kidneys, suprarenal glands, aorta, IVC) that never had a mesentery, shown with green tint against the posterior wall. Panel C: Secondary retroperitoneal organs (pancreas, duodenum, ascending and descending colon) that fused with the posterior wall during development, shown with yellow tint. Panel D: Complete sagittal section demonstrating the relationship of all organs to the peritoneum with labeled mesenteries containing vessels and 5 cm scale bar. --- ## Peritoneal Folds The peritoneum forms several types of folds that connect organs to the body wall or to each other, transmitting blood vessels, nerves, and lymphatics. Mesenteries are double layers of peritoneum that suspend organs from the body wall. The mesentery of the small intestine, often simply called "the mesentery," suspends the jejunum and ileum from the posterior abdominal wall. Its root extends obliquely from the left of the L2 vertebra to the right iliac fossa, spanning approximately 15 centimeters. Between its layers run the superior mesenteric vessels and associated lymph nodes. The transverse mesocolon suspends the transverse colon from the posterior abdominal wall. Its root runs along the anterior surface of the pancreas. The middle colic vessels travel within this fold. The sigmoid mesocolon suspends the sigmoid colon and has an inverted V-shaped root. The sigmoid and superior rectal vessels course within it. The mesoappendix is a small triangular fold suspending the appendix from the mesentery of the ileum. The appendicular artery, a branch of the ileocolic artery, travels in its free edge. The omenta are specialized peritoneal folds associated with the stomach. The greater omentum is often called the "abdominal policeman" because it migrates to sites of infection or inflammation, helping to wall off pathology and limit its spread. This four-layered structure hangs from the greater curvature of the stomach, drapes over the transverse colon and small intestine like an apron, and attaches to the transverse colon. It contains abundant fat and lymphoid tissue with immune cells. The lesser omentum is a double layer extending from the liver to the stomach and first part of the duodenum. It has two named portions: the hepatogastric ligament connects the liver to the lesser curvature of the stomach, while the hepatoduodenal ligament connects the liver to the first part of the duodenum. The free right edge of the lesser omentum, the hepatoduodenal ligament, contains the portal triad: the hepatic artery proper, portal vein, and bile duct. Panel A: The greater omentum as a fatty, apron-like four-layered structure hanging from the greater curvature of the stomach and draping over the intestines. Panel B: The lesser omentum extending from the liver to the stomach and duodenum, with the hepatoduodenal ligament at its free right edge containing the portal triad. Panel C: The transverse mesocolon suspending the transverse colon from the posterior wall along the anterior surface of the pancreas. Panel D: The fan-shaped mesentery of the small intestine suspending jejunum and ileum from its 15 cm root, containing superior mesenteric vessels with 5 cm scale bar. --- ## Peritoneal Ligaments Peritoneal ligaments are double layers of peritoneum that connect organs to the body wall or to each other. These are not true ligaments in the musculoskeletal sense but rather peritoneal reflections. The liver has several peritoneal ligaments. The falciform ligament attaches the liver to the anterior abdominal wall and diaphragm, extending from the umbilicus upward. It contains the ligamentum teres, the obliterated remnant of the umbilical vein from fetal life. The coronary ligament attaches the liver to the diaphragm and encloses the bare area of the liver, the portion not covered by peritoneum. The right and left triangular ligaments represent the lateral extensions of the coronary ligament. The hepatogastric and hepatoduodenal ligaments, parts of the lesser omentum, connect the liver to the stomach and duodenum respectively. The stomach has several peritoneal connections. The gastrophrenic ligament connects the fundus of the stomach to the diaphragm. The gastrosplenic ligament runs from the greater curvature of the stomach to the spleen, transmitting the short gastric vessels and left gastroepiploic vessels. The gastrocolic ligament, part of the greater omentum, connects the stomach to the transverse colon. The spleen is connected by two ligaments. The gastrosplenic ligament from the stomach contains the short gastric and left gastroepiploic vessels. The splenorenal (or lienorenal) ligament connects the spleen to the left kidney and contains the splenic vessels and the tail of the pancreas. Panel A: The falciform ligament extending from the anterior abdominal wall to the liver, containing the ligamentum teres (obliterated umbilical vein) in its free edge. Panel B: The coronary ligament attaching the liver to the diaphragm with anterior and posterior layers enclosing the bare area, and triangular ligaments at the lateral extents. Panel C: The gastrosplenic ligament connecting the stomach to the spleen and transmitting the short gastric and left gastroepiploic vessels. Panel D: The splenorenal ligament extending from the spleen to the left kidney, containing the splenic vessels and tail of the pancreas, with all ligaments labeled and 4 cm scale bar. --- ## Divisions of the Peritoneal Cavity The peritoneal cavity is divided into two interconnected compartments: the greater sac and the lesser sac. The greater sac is the main peritoneal cavity, containing most of the abdominal organs. It communicates with the lesser sac through a single opening, the epiploic foramen. The lesser sac, also called the omental bursa, is a smaller compartment located behind the stomach and lesser omentum and in front of the pancreas. It is an important surgical space that provides access to the posterior surface of the stomach and the anterior surface of the pancreas. The lesser sac is bounded anteriorly by the stomach and lesser omentum, posteriorly by the pancreas, left kidney, and left suprarenal gland, superiorly by the caudate lobe of the liver, inferiorly by the transverse colon and transverse mesocolon, on the right by the epiploic foramen, and on the left by the spleen and its ligaments. The epiploic foramen, also known as the foramen of Winslow, is the sole communication between the greater and lesser sacs. It lies behind the free edge of the lesser omentum, specifically the hepatoduodenal ligament. The foramen is bounded anteriorly by the hepatoduodenal ligament containing the portal triad, posteriorly by the inferior vena cava and right crus of the diaphragm, superiorly by the caudate lobe of the liver, and inferiorly by the first part of the duodenum. Panel A: The greater sac as the main peritoneal cavity containing the bowel loops and most abdominal organs anteriorly. Panel B: The lesser sac (omental bursa) as a smaller compartment behind the stomach and lesser omentum, anterior to the pancreas. Panel C: The epiploic foramen (foramen of Winslow) as the sole communication between the sacs, bounded by the hepatoduodenal ligament anteriorly and IVC posteriorly. Panel D: Transverse section at the foramen level showing the caudate lobe superiorly, portal triad within the hepatoduodenal ligament, and arrows indicating the sac communication with 3 cm scale bar. --- ## Peritoneal Recesses and Gutters Several recesses and potential spaces within the peritoneal cavity are clinically significant as sites where fluid, blood, or pus can accumulate. The subphrenic recesses lie between the diaphragm and the superior surface of the liver. The right and left subphrenic spaces are separated by the falciform ligament. Abscesses commonly form in these spaces following surgery or visceral perforation, and patients may experience referred pain to the shoulder due to diaphragmatic irritation of the phrenic nerve. The subhepatic recess lies between the inferior surface of the liver and the structures below it. The deepest part of the upper abdomen when a patient is supine is the hepatorenal recess, also known as Morrison's pouch. This space lies between the right lobe of the liver anteriorly and the right kidney posteriorly. Fluid from anywhere in the abdominal cavity gravitates to this location in a supine patient, making it an important site to examine on ultrasound for free abdominal fluid. The paracolic gutters are the spaces lateral to the ascending and descending colon. The right paracolic gutter is continuous with the pelvis below and the subhepatic space above, allowing free communication of fluid between these regions. The left paracolic gutter is partially blocked superiorly by the phrenicocolic ligament, which extends from the splenic flexure of the colon to the diaphragm. This limits upward spread of infection on the left side. The pelvic recesses represent the lowest points of the peritoneal cavity. In males, the rectovesical pouch between the rectum and bladder is the most dependent point. In females, two pouches exist: the vesicouterine pouch between the bladder and uterus, and the rectouterine pouch (pouch of Douglas) between the rectum and uterus. The rectouterine pouch is the lowest point in the female peritoneal cavity and is clinically accessible through the posterior vaginal fornix for drainage or aspiration. Panel A: The right and left subphrenic spaces below the diaphragm and above the liver, separated by the falciform ligament, where abscesses commonly form. Panel B: Morrison's pouch (hepatorenal recess) between the liver and right kidney as the deepest point in the supine upper abdomen where fluid collects. Panel C: The paracolic gutters lateral to the ascending and descending colon, with the phrenicocolic ligament blocking superior spread on the left side. Panel D: Pelvic recesses including the rectovesical pouch in males and the vesicouterine and rectouterine (pouch of Douglas) pouches in females as the lowest peritoneal points, with 5 cm scale bar. --- ## Clinical Significance of Peritoneal Spaces Understanding peritoneal spaces has important clinical applications for predicting the spread of pathology and planning interventions. Infected or contaminated fluid spreads along predictable pathways based on gravity and anatomical barriers. The right paracolic gutter provides a direct route from the pelvis to the subhepatic space. Pus from a ruptured appendix, for example, may track up the right gutter and accumulate in the subhepatic space, potentially forming a subphrenic abscess. The phrenicocolic ligament limits similar spread on the left side. Ascites, the accumulation of fluid in the peritoneal cavity, occurs in conditions such as liver cirrhosis, heart failure, malignancy, and peritoneal infection. On physical examination, ascites produces shifting dullness to percussion and a fluid wave. The fluid accumulates first in the dependent recesses, particularly the pelvic pouches and Morrison's pouch. Paracentesis is the procedure for draining peritoneal fluid. The preferred insertion site is typically the left lower quadrant to avoid the cecum on the right and the inferior epigastric vessels near the midline. Ultrasound guidance improves safety and success. Peritoneal dialysis exploits the large surface area and absorptive capacity of the peritoneum. A catheter is inserted into the peritoneal cavity, and dialysate fluid is instilled. Waste products diffuse from the blood across the peritoneum into the fluid, which is then drained. The peritoneum can absorb approximately 35 milliliters of fluid per hour, enhanced by diaphragmatic movement during respiration. Panel A: Supine patient showing fluid tracking from the pelvis up the right paracolic gutter to Morrison's pouch and subhepatic space, with the phrenicocolic ligament blocking leftward spread. Panel B: Ascites accumulation in dependent spaces including the pelvis, paracolic gutters, and subhepatic region, demonstrating shifting dullness pattern. Panel C: Paracentesis technique with needle entering the left lower quadrant peritoneal cavity, avoiding the cecum and inferior epigastric vessels. Panel D: Peritoneal dialysis showing catheter placement, dialysate filling the peritoneal cavity, and diffusion of waste products across the large peritoneal surface with 5 cm scale bars. --- ## Blood Supply and Lymphatics The blood supply to the peritoneum follows its parietal and visceral distribution. The parietal peritoneum receives arterial blood from the body wall arteries, including branches of the musculophrenic, deep circumflex iliac, lumbar, and iliac arteries. Venous drainage follows corresponding veins to the systemic circulation. The visceral peritoneum shares its blood supply with the organs it covers, receiving branches from the celiac trunk, superior mesenteric artery, and inferior mesenteric artery. Venous blood from the visceral peritoneum drains into the portal system. The peritoneum has an extensive lymphatic network that provides important pathways for immune surveillance and, unfortunately, for the spread of malignancy. Peritoneal carcinomatosis, the widespread seeding of cancer cells throughout the peritoneal cavity, represents a common pattern of spread for ovarian, gastric, colon, and pancreatic cancers. The peritoneum can absorb fluids, drugs, and toxins at a rate of approximately 35 milliliters per hour under normal conditions. Absorption is enhanced by diaphragmatic movement and the respiratory pump, which draws fluid into the lymphatics. --- ## Nerve Supply The innervation of the peritoneum has critical clinical implications for understanding abdominal pain. The parietal peritoneum lining the anterior and lateral abdominal walls receives somatic innervation from the intercostal nerves T7 through L1. Pain arising from this region is sharp, well-localized, and perceived at the site of irritation. The central diaphragmatic parietal peritoneum is innervated by the phrenic nerve (C3-5), so irritation here causes referred pain to the shoulder via the shared cervical dermatomes. The pelvic parietal peritoneum receives innervation from the obturator nerve, and irritation may cause referred pain to the medial thigh. The visceral peritoneum receives autonomic innervation from both sympathetic and parasympathetic sources. This layer is sensitive to stretch and distension but not to cutting or burning. Pain from visceral peritoneum is dull, poorly localized, and typically perceived in the midline because of bilateral autonomic innervation. The classic pain pattern of acute appendicitis illustrates these principles. Early in the disease, when only the visceral peritoneum of the appendix is irritated, pain is poorly localized and perceived in the periumbilical region (visceral pain referred to the T10 dermatome). As inflammation progresses to involve the parietal peritoneum overlying the appendix, pain becomes sharp and localizes to the right lower quadrant (somatic pain from direct parietal irritation). Panel A: Dermatome bands T7-L1 on the anterior abdominal wall corresponding to the somatic innervation of the parietal peritoneum for well-localized pain. Panel B: Shoulder pain referral from central diaphragmatic irritation via the phrenic nerve sharing C3-5 dermatomes with the supraclavicular region. Panel C: Visceral peritoneum with autonomic innervation producing poorly localized midline pain from bowel distension or inflammation. Panel D: Appendicitis pain migration demonstrating initial periumbilical visceral pain (T10) progressing to localized right lower quadrant somatic pain when parietal peritoneum becomes involved, with 5 cm scale bar. --- ## Clinical Correlations Peritonitis is inflammation of the peritoneum, representing one of the most serious abdominal emergencies. It can result from perforation of a hollow viscus (appendicitis, peptic ulcer, diverticulitis), trauma, or spread from an infected organ. Patients present with severe abdominal pain, rigidity of the abdominal wall, rebound tenderness, and guarding on examination. Peritonitis may be generalized or localized depending on the cause and whether the greater omentum has been able to wall off the process. Subphrenic abscess is a collection of pus below the diaphragm, typically following surgery or perforation of an abdominal viscus. Patients present with fever and may experience referred pain to the shoulder from diaphragmatic irritation. Imaging with CT scan confirms the diagnosis, and treatment involves drainage and antibiotics. Hernias form when abdominal contents protrude through weaknesses in the abdominal wall, and the peritoneum forms the hernia sac. Internal hernias occur through openings within the peritoneal cavity, such as the epiploic foramen or defects in mesenteries. External hernias protrude through defects in the abdominal wall to lie superficial to the body wall. The Pringle maneuver is a surgical technique that exploits the anatomy of the hepatoduodenal ligament. By compressing the free edge of the lesser omentum, the surgeon occludes the portal triad (portal vein, hepatic artery, and bile duct), thereby controlling hemorrhage from the liver. This maneuver helps determine whether liver bleeding is from the hepatic vasculature (which would stop with the maneuver) or the hepatic veins or inferior vena cava (which would continue). Panel A: Peritonitis presenting with rigid, tender abdomen and guarding, with an inset showing inflamed hyperemic peritoneum causing the clinical findings. Panel B: Subphrenic abscess as a collection below the right hemidiaphragm causing fever and referred shoulder pain from diaphragmatic irritation. Panel C: The Pringle maneuver compressing the hepatoduodenal ligament at the free edge of the lesser omentum to control hepatic hemorrhage. Panel D: Close-up of the portal triad (hepatic artery, portal vein, bile duct) being compressed during the Pringle maneuver, demonstrating hemorrhage control technique with 3 cm scale bars. --- ## Summary The peritoneum is a serous membrane with parietal and visceral layers separated by a potential cavity containing lubricating fluid. Intraperitoneal organs are mobile and suspended by mesenteries, while retroperitoneal organs are fixed to the posterior abdominal wall with only their anterior surface covered. The greater omentum serves as the "abdominal policeman," migrating to sites of infection to wall off pathology. The lesser sac lies behind the stomach and communicates with the greater sac through the epiploic foramen. Morrison's pouch (the hepatorenal recess) is the deepest space in the supine upper abdomen, while the pelvic pouches (rectovesical in males, rectouterine in females) are the lowest points of the peritoneal cavity. The paracolic gutters allow fluid spread, with the right gutter communicating freely from pelvis to subhepatic space while the phrenicocolic ligament limits spread on the left. The parietal peritoneum has somatic innervation producing well-localized pain, while the visceral peritoneum has autonomic innervation producing poorly localized midline pain. This explains the classic pain progression of appendicitis from periumbilical to right lower quadrant localization. --- ## Key Terms | Term | Definition | |------|------------| | Mesentery | Double layer of peritoneum suspending an organ from the body wall and transmitting vessels and nerves | | Greater omentum | Four-layered fatty apron extending from the stomach to the transverse colon; contains immune cells and walls off infection | | Lesser sac | Peritoneal compartment behind the stomach, communicating with the greater sac through the epiploic foramen | | Epiploic foramen | Opening between the greater and lesser sacs, bounded by the hepatoduodenal ligament anteriorly and IVC posteriorly | | Morrison's pouch | Hepatorenal recess; the deepest point in the supine upper abdomen where fluid collects | | Pouch of Douglas | Rectouterine pouch; the lowest point in the female pelvis, accessible through the posterior vaginal fornix | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*